Method of inhibiting blistering in heat-treating aluminum alloys



Patented May 21, 1946 tion of Delaware METHOD OF IN HIE T-TREATINGGharles Beneke, Louisvill Reynolds Metals 60.,

ATENT orrice rrmc 'nms'runmd' IN ALUMINUM ALLOYS Ky d to Richmond, Va 2corpora- No Drawing. Application October 30, 1943, Serial No. 503,334

1 Claim.

. this heat treatment the said aluminum prod ucts are subject toblistering, through a phenomenon which is termed high temperatureoxidation and which has in the past been considered as due to emanationsfrom the fuel in the furnace, if it is a combustion furnace, fromatmospheric conditions of theair itself, or from other causes as yetundetermined. An aluminum alloy which is particularly subject toblistering under heat treatment is known as 14-8, 9. second alloy beingknown as 17-8; both of these alloys being of relatively high coppercontent. v

The blistering is of several types, and in a considerable proportion ofcases the blistering extends internallyfor a, substantial distance. Instill other cases the blistering extends only a fraction of an inchinwardly of the surface.

The problem of blistering has been particularly felt in aluminum alloyforgings used in the aircraft industry, wherein they result of defectiveparts may be Very serious. In fact, inspection of forgings does notalways bring to light internal defects due to blistering and from whichforgings superficially appearing blisters have been removed by grindingor machining.

Due to the blistering problem, in the heattreatment of bar stock andother aluminum stocks, and in the heat treatment of forged aluminumalloys, considerable loss has occurred in manufacturing operations,because even where the product can be used by removing surface blisters,and bar stock for example, may partiallybe utilized by cutting outblistered sections, the loss nevertheless is substantial.

It has been proposed to coat the said aluminum alloys, prior to heattreatment, with a coating of sodium fiuoborate in order to reduceblistering, but there are objections thereto and such coatings are noteffective in oil-fired furnaces.

. I have discovered a method of treating such hard aluminum alloys priorto heat treatment so that blistering is inhibited during the heattreating operations, and my method may be employed with gas-firedfurnaces and all other types of heat treating apparatus.

The principle of my blister inhibiting treatment is that it minimizeshigh temperature oxidation of the alloy, particularly near the surface,and provides the sorbing characteristic My treatment is particularlyadvantageous in furnaces where products of combustion come in contactwith the aluminum products being treated, i. e., oil-fired,and gas-firedfurnaces of of immersion is a types of furnaces and num alloys, the

the recirculating type.

After a heat treating operation the coating provided by my treatment maybe quickly removed and in such manner that the aluminum product is in anideal surface condition for thorough inspection.

As an example of my method, the aluminum alloy product to be heattreated is given the following treatment prior to the heat treatingprocess:

A bath consisting of a caustic soda solution, i. e., a. sodium hydroxidesolution, is prepared, the concentration of caustic being preferably inthe ratio of 8%-15%. The aluminum alloy products are immersed in thecaustic solution for sufficient time to form thereon an oxide coating.When the aluminum alloys contain a substantial proportion of copper, asin the case of I l -S aluminum alloy, for example, the oxide coatingwill consist of aluminum oxide and cupric oxide. The surface of theproduct so treated will approach the color of a. black body, andgenerally the time matter of minutes, it only being required that thecoating be homogeneously formed throughout the surfaces of the aluminumalloy products.

When the coating has been formed the products are withdrawn from thebath (which bath may be operated at a temperature of about F.) theproducts then being washed, preferably by ahot water rinse.

when the coated surfaces of the product'are dried the products may beheat-treated in all under the customary heat treating temperatures forthese specific alumifurnaces including oil-fired furnaces. The productswhen thus treated inhibit blistering.

Throughout the heating treating operation the dark coated surfacesremain on the said aluminum alloy products. After the heat-treatingoperations the said aluminum alloy products are removed from the heattreating furnaces, and are alloy with a higher heat abin a given lengthof time.

nitric acid solution,

mersing the said products in an etching bath of say of 15-20%concentration, then removed from the etching bath and subjected to acold water 'rinse.

By means of my. method alloy products are protected against blisteringunder all conditions of heat treatment andthe great loss heretoforeoccurring is prevented.

Having desc bed my invention, what I claim and desire to secure byLetters Patent is as ,follows:

the said aluminum 2 2,400,804 quenchedin accordance with customarypractice. A method of inhibiting blistering in heat Thereupon, thecoatingsmay be removed by imtreatment an alloy predominately of aluminumand containing minor proportion alloying metals exclusively of ironexcept as an impurity, which comprises immersing said product in a bathof caustic soda of 8 to 15% concentration whereby a homogeneous darkcoating is formed thereon,

and subsequently water to remove the .excess alkali.

CHARLES BENEKE.

washing said coating with hot

